KINSHASA, Nov 17 (Reuters) - U.N. attack helicopters hit rebel positions in eastern Congo on Saturday after insurgents gained ground in heavy fighting with government troops a n d took control of a town, the United Nations said.

The clashes to the south of the town Kibumba mean the rebels have advanced to within 30 km (18 miles) of Goma, the closest they have been to North Kivu's provincial capital since a rebellion exploded in the eastern provinces eight months ago.

North Kivu Governor Julien Paluku said the army retreated to the southern outskirts of the town after M23 rebels - a group of soldiers who mutinied in April - advanced with support from neighbouring Rwanda. A Congolese government statement said 4,000 Rwandans had crossed the border, although Kinshasa later reduced that estimate to 3,500.

Rwanda rejected the accusations, the latest in a string of charges by the Congolese government in Kinshasa. The Rwandan government called on Congo's army and the rebels to halt the fighting as shells were landing in its territory.

"Kibumba has fallen into the hands of the M23," a spokesman for the U.N. Department of Peacekeeping Operations said in an email. "Latest reports indicate that the FARDC (Congo's army) and MONUSCO (U.N.) forces are attempting to hold off a possible M23 advance toward Goma at Kibati, some 20 km north of Goma."

"We are not in a position to confirm direct Rwandan involvement in the M23 attacks," the spokesman said. "However, we are very concerned by reports that the M23 attacking forces appear to be well-equipped and supplied."

The spokesman added that U.N. Secretary-General Ban Ki-moon called Congolese Foreign Minister Raymond Tshibanda on Saturday to voice support for Kinshasa, and Rwandan President Paul Kagame to urge him to "use his influence on the M23 to help calm the situation and restrain the M23 from continuing their attack."

The French U.N. mission in New York called an emergency closed-door meeting of the Security Council on the escalating violence.

"I do hope that we will get a signal from the council telling the M23 to stop its advance and ... to call on all the countries concerned to enter into a real negotiation," French Ambassador Gerard Araud told reporters before the meeting.

"There won't be any solution without an agreement between the DRC and its neighbors, including Rwanda," he said.

'DEVASTATING' HUMANITARIAN IMPACT

More than 5 million people are estimated to have died from violence, hunger and disease in wars in Congo since 1998, which would make it the deadliest conflict since World War Two.

The peacekeeping spokesman said the humanitarian impact of the fighting was "devastating." He added that U.N. troops in Goma and at its airport were on "high alert."

The United Nations has about 6,700 MONUSCO forces in North Kivu, with about 1,400 troops in Goma and the surrounding area.

U.N. experts have said in reports they have evidence that Rwanda has supported the M23 rebels in eastern Congo. They have called on the Security Council to impose sanctions on Rwandan officials in response.

"The Rwandan army came across the border behind our troops, that's why our troops withdrew," Paluku told Reuters by telephone.

"The (rebels) are just a few kilometres away, so of course Goma is under threat, we can't hide that," he said, adding that government troops were regrouping at Kilimanyoka, 12 km (7 miles) north of the city. Rebels said they had no immediate plans to attack Goma.

The U.N. peacekeeping mission in Congo said army units had come under heavy weapons fire since early on Saturday morning, forcing civilians to flee. U.N. attack helicopters were dispatched to strike rebel positions south of Kibumba.

"So far ten missions have been carried out by our attack helicopters," the U.N. said in a statement. The U.N. has a mandate to protect civilians and support government troops when they need it.

No casualty figures have been given by any force.

Rwanda's army has repeatedly sent soldiers into Congo during nearly two decades of conflict in Africa's Great Lakes region, but Kigali has strongly denied Congolese and U.N. accusations of support for the M23.

"These are absolutely false allegations. They are very tired, and very old. Whenever DRC (the Democratic Republic of Congo) is defeated on the battlefield, it's meant to be (Rwanda's army)," Rwandan army spokesman Brigadier General Joseph Nzabamwita told Reuters.

"Rwanda has called on (Congo's army) and M23 to stop this useless war ... Rwanda is being violated by constant bomb shells on our territory," he added.

More than three-quarters of a million people have fled their homes since the fighting began, and regional efforts to find a solution have so far failed.

M23 spokesman Vianney Kazarama told Reuters the rebels were now in control of Kibumba, but said they would not advance further.

"We're stopping here, we're waiting, we're not going to Goma," he said, reiterating a call for the government to start negotiations. (Additional reporting by Jenny Clover in Kigali, Louis Charbonneau in New York; Writing by David Lewis; Editing by Rosalind Russell and Peter Cooney)

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